The present invention relates to the techniques for reinforcing pipelines and, in particular, to the repair of underground pipelines.
These pipelines are, for example, part of a water supply network. In their commonest form, these pipelines consist of an end-to-end assembly of prestressed concrete pipe segments with a diameter ranging from about 0.5 to 6 meters. Water circulates in them at a pressure which may be about twenty bars. These concrete pipe segments may comprise an inner metal liner. However, the latter is not designed to withstand the pressure forces due to the circulating fluid. These forces are absorbed by the prestressing applied to the concrete by means of wires wound spirally on the outside of the concrete core of the pipe segment. During the prefabrication of such segment, its concrete core is set in rotation about its axis in order to receive the wire which is braked for the purpose of tensioning. This wire is subsequently protected against corrosion by spraying an additional layer or concrete or mortar over a few tens of millimeters.
In spite of this protecting layer and of the passivation of the steel forming these prestressing wires, it may happens that the prestressing wires experience corrosion which generally leads to a deterioration in the protective coating and therefore to an acceleration of the phenomenon which may cause a break of the wires and therefore a structural embrittlement of the pipeline or even the fracture of the latter under the effect of the pressure of the circulating fluid.
After the damage to one or more segments has been detected by preventive methods (in particular, by acoustic detection), the traditional methods involve first excavating the material (sand, earth or the like) surrounding the damaged segments, then either simply replacing the segments in question, which necessitates draining the supply network beforehand, thus resulting in a considerable detriment to the operation of the network, or manually repairing the outer surface of the pipeline, if the latter has not yet given way.
International Patent Application No. WO 03/014614, the content of which is incorporated herein by reference, discloses a method for repairing a pipeline for the supply of a fluid under pressure, composed of a succession of underground prestressed concrete pipe segments, comprising the steps of:                making a trench in order to free a length of the pipeline;        carrying out a first excavation in the trench under a first pipeline portion substantially centered on a junction between two adjacent segments;        installing and tensioning at least one prestressing reinforcement around the first portion of the pipeline;        filling in the first excavation with soil material and compacting this material;        carrying out a second excavation in the trench under a second pipeline portion adjacent to the first portion and included in a single segment;        installing and tensioning at least one prestressing reinforcement around the second portion of the pipeline; and        filling in the second excavation with soil material and compacting this material.        
This method has the advantage of making it possible to restore a damaged zone of the pipeline, whilst at the same time adhering to the technical constraints associated with this type of structure and its operation.
On this type of large pipeline, there are generally manholes uniformly spaced apart over its length, enabling an operator to have access to equipment such as valves, pressure control devices, vents or the like, or to enter inside the pipeline to carry out inspection or repair work in there. Such a manhole gives rise to a radial projection on top of the pipeline. Where it enables access to the interior of the pipeline, the projection contains a lock chamber having an upper hatch for access from outside and a lower hatch equipped with safety valves.
The manhole zones are preferential corrosion points, since the original prestress provided by wires has to be interrupted there and anchored by means of studs between large steel plates. In addition, it is not rare that water is present at the base of the manhole.
FIG. 1 shows such a manhole projection 1 on top of a pipeline 2, around which prestressing reinforcements 3 have been installed, for example according to the method described in the above-mentioned International Patent Application No. WO 03/014614. It can be seen that the additional prestressing reinforcements cannot surround the pipeline in the manhole zone 4.
An object of the present invention is to make it possible to complete the installation of prestressing reinforcements around a pipeline, including in the zones having projections, such as those required for manholes.